Abstract

Perovskite-based materials are in the focus of research not only because of their excellent
physical properties, but also because their relatively simple structure facilitates
the understanding of structure-property relationships, which is crucial for developing
novel materials with improved qualities. Recent research in the field of ferroelectric
and piezoelectric materials is concerned with the development of eco-friendly lead-free
materials. To achieve this goal, it is important to understand the fundamental correlation
between the ‘Structure’ and the ‘Property’. In this work, the primary focus has
been to elucidate the structural changes occurring as a function of doping in three different
systems: (1) BiScO3-PbTiO3 (BS-PT), a recently developed system which has
already attracted much interest because of its superior physical properties near the morphotropic
phase boundary (MPB); (2) BiScO3-BaTiO3 (BS-BT), which can be considered
as a lead-free analogue of the BS-PT family and lastly, (3) Na0.5Bi0.5TiO3-BaTiO3
(NBT-BT), which is a well-known lead-free material at the NBT-rich side of the phase
diagram.
Powder samples with a range of compositions for each system were prepared following
the solid-state synthesis route and were investigated utilizing both neutron and
x-ray powder diffraction and dielectric measurements. Detailed crystallographic information
was obtained by Rietveld refinement against the neutron powder diffraction
data. Structural phase transitions as a function of temperature were determined by nonambient
x-ray powder diffraction and compared with the physical properties of the ceramics
using high-temperature dielectric measurements. The significant outcomes are:
1. The best model to represent the so-called MPB of xBS-(1-x)PT system is found to
be a mixture of a tetragonal and a monoclinic phases from the powder diffraction
data. The structure beyond the MPB compositions is in better agreement for a
single monoclinic model with the space group Cm than the accepted space group
R3m. By contrast, single crystals with compositions around the MPB provide
evidence for a model consisting of two primitive monoclinic cells.
2. The lead-free BS-BT system exhibits an extended phase boundary between tetragonal
and pseudocubic phases, which can be modelled by a combination of tetragonal
and rhombohedral phases. The incorporation of BS into BT also results in
the suppression of the two low-temperature phase transitions of BT.
3. Samples with new compositions synthesized in the xNBT-(1-x)BT system demonstrate
a rare enhancement in the tetragonality of the unit cell and an increase in
the Curie temperature for compositions where x <= 0.40.